Adaptive traction control system

ABSTRACT

A method of controlling a traction control system ( 30 ) includes continuously adapting a steady state driven wheel speed to reference wheel speed ratio, so that said traction control system can avoid unnecessary actuations (e.g., demanding torque reduction). The continuous adaptation methodology provides traction control robustness to vehicles equipped with a spare tire, or a different final drive such as in the use of aftermarket parts. The method includes a dual rate adaptation that allows both fast adaptation and fine tuning capabilities of the ratio. The method includes comparing the instant driven wheel speed to reference wheel speed ratio to the filtered driven wheel speed to reference wheel speed ratio, to obtain a ratio difference. When the difference is above a threshold, the first filter constant is selected and the first constant is applied to an adaptation filter, resulting in a first filtered and adapted ratio. The traction control system is controlled with the adapted ratio. When the difference is below the threshold, the second filter constant is selected and the selected constant is applied to the adaptation filter, resulting in the second filtered and adapted ratio. The traction control system is controlled with the adapted ratio.

TECHNICAL FIELD

The present invention relates generally to a traction control system,and more particularly, to a drive control system that adapts to changedvehicle components such as a mini spare tire.

BACKGROUND

Traction control systems generally use braking and engine interventionto reduce wheel slip at the driven wheels of the vehicle. The non-drivenwheel speeds provide a source for a reference speed of the vehicle. Incurrent systems the assumption is that both the driven and non-drivenwheels have a same or similar tire rolling radius. In certainsituations, a customer may modify the wheel or tire size significantly.One example of a customer modification is the use of a mini spare tire.When using a mini spare tire, a traction control system may falselyactivate. Also, customers, particularly in trucks, may modify thedifferential, which in turn may falsely activate the traction controlsystem.

It would therefore be desirable to automatically adjust the tractioncontrol system to reduce the amount of false activations.

SUMMARY

The present invention improves a traction control system by allowing thetraction control system to adjust to a change in the rolling radius of avehicle such as when a wheel has been replaced by a mini spare. Thepresent invention also allows a modification of the driven to non-drivenwheel ratio in the case when a differential has been modified by thecustomer.

In one aspect of the invention, a method for controlling a tractioncontrol system includes determining a reference speed, determining adriven wheel speed, and determining a long term filter value. The methodfurther includes determining a long term wheel speed ratio from thereference speed, the driven wheel speed, and the long term filter. Ashort term wheel speed ratio is determined from the reference speed andthe driven wheel speed.

One advantage of the invention is that when a customer installs a wheelwith a different radius, such as a spare tire or custom wheels, or thecustomer installs a different differential, the traction control systemrapidly adapts to prevent false activations.

Other advantages and features of the present invention will becomeapparent when viewed in light of the detailed description of thepreferred embodiment when taken in conjunction with the attacheddrawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagrammatic view of a vehicle having a tractioncontrol system according to the present invention.

FIG. 2 is a flow chart illustrating the operation of the presentinvention.

FIG. 3 is a flow chart illustrating the operation of a watchdog for theflow chart illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following figures describe a traction control system that may beused for an engine only traction control system. However, the teachingsof the present invention may also be applicable to engine and brake-typetraction control systems.

An automotive vehicle 10 is illustrated having wheel and tire assemblies12 a, 12 b, 12 c, and 12 d. A drive system 14 coupled to a drivetrain 16provides power to the wheel and tire assemblies 12 through the frontdifferential 18 and rear differential 20. The drive system 14 mayinclude an engine controller that is microprocessor-based. The enginecontroller may be used to control the amount of torque and thus theamount of slip generated at each of the wheels. The drive system 14 mayvary the amount of engine power to effect the change in torque at thewheel and tire assemblies 12. A reduction in the amount of fuel andchanging other parameters may reduce the amount of power output from theengine. It should also be noted that the drive system may include aninternal combustion-only type system, a hybrid system, an electric drivesystem, or a fuel cell system.

The drive system 14 provides torque through the drivetrain 16 which mayinclude a transmission, front differential 18 and rear differential 20.In a rear wheel drive vehicle, only the rear differential 20 would bepresent. A four or all-wheel drive system may include both the frontdifferential 18 and the rear differential 20. Also, in certain all-wheeldrive systems an additional center differential may be provided.

A brake system 24 that may include electric, electro-hydraulic, orhydraulic systems is used to actuate the brakes 26 a, 26 b, 26 c, and 26d. The brakes are activated in response to driver input. The brakesystem 24 may also be activated in response to a traction control systemthat include a traction control controller 30.

The traction control system and thus the controller 30 may be coupleddirectly or indirectly to wheel speed sensors 32 a, 32 b, 32 c, and 32d. As illustrated, the wheel speed sensors are coupled directly to thewheels. The wheel speed sensors may be the output from an anti-lockbrake system, an axle sensor or the like.

It should be noted that the wheels driven by the drive system 14 arereferred to as driven wheels whereas wheels that are not coupled to theengine are referred to as undriven wheels. In the following descriptionthe speed of the undriven wheels may be used as the reference speed. Infour-wheel or all-wheel drive systems no wheels are undriven and thusvarious algorithms may be used to determine the reference speed. Thereference speed may correspond to a vehicle speed. Wheels that aretouching the road surface and have a torque level below a certain torquethreshold may be included in such algorithms.

The traction controller 30 is coupled to the drive system 14. Thetraction controller 30 may generate a control signal or torque commandto reduce the amount of torque to the engine upon the sensing of aslipping wheel.

Referring now to FIG. 2, a method for operating a traction controlsystem is set forth. In step 50 it is determined whether the tractioncontrol system has been activated. If the traction control system hasbeen activated, determining the wheel speeds and the like may beaffected by the system and thus not reliable. Therefore, the processends in step 52. In step 54, the reference speed and the wheel speedsare determined. The reference speed as mentioned above may be derivedfrom the underdriven wheels or from an algorithm determining the vehiclespeed. The wheel speeds of the driven wheels are also determined. Instep 56, when the wheel speeds are not above a threshold, step 52 endsthe process. That is, when a sufficient amount of speed is not providedin the vehicle, the system may not be activated. In step 56, if thewheel speeds are above a threshold, step 58 determines whether thesignals are noisy, i.e. if the high frequency content of the signals issignificant or above a noise threshold. If the signals are noisy, aproper signal to noise ratio for an accurate determination may not bepresent. Therefore, step 52 is executed when the signals are noisy.

In step 60, an instant ratio of the driven to non-driven wheels or thedriven wheels to a reference speed is determined. That is, the currentratio between the non-driven and driven wheels may be determined. Theratio may include the ratio of the wheel speeds which ultimately may beused to determine the ratios and the rolling radius or the drivetrain ordifferential gear ratios.

In step 62, the instant ratio is compared to the dual rate filteredratio that is constantly being adapted. In step 64, the instant and thelong term or dual rate filtered ratios are compared. When the differenceis greater than a certain percentage or a certain amount, both of whichmay be referred to as a threshold, step 66 is performed. In step 66,fast adaptation filter constant is used when the amount is greater thanthe threshold. In step 64, if the difference is less than the threshold,step 68 uses a slow adaptation and thus a slow adaptation filterconstant is used. In this embodiment various filtering schemes such asaveraging may be provided over the long term so that the values of theratio slowly change. Such conditions that slowly change include tirepressure loss and tread wear which slowly change the rolling radius ofthe tire and wheel assembly. Steps 66 and 68 are used to apply anadapted speed ratio output in step 71. The adapted ratio output may berepresentative of the rolling radius ratio and the differential gearratio of the vehicle. The final ratio output may be the driven tonon-driven wheel output in a two-wheel drive vehicle, and driven to areference vehicle velocity in a four-wheel drive or all-wheel drivevehicle. In step 72 the traction control system is controlled inresponse to the final ratio output. This allows the differences inrolling radius and differential gear ratios to be accommodated in thesystem to prevent false activations. As will be appreciated, the aboveprocess may be performed continuously so that the ratio is constantlyadapted when the conditions of steps 50-58 are met.

Referring now to FIG. 3, in certain situations the traction controlsystem may not catch various conditions soon enough. Therefore, in step80 a watchdog for the entire system is used. In step 82 the durationthat the traction control is operated is compared to a threshold T₀.Also, the driven wheel speed W_(driven) is compared to a factormultiplied by the target wheel speed. If the driven wheel speed is abovethe R times W_(target) continuously for a time threshold T₁, instantadaptation may be performed in step 84. The watchdog essentially islooking for conditions that are not physically plausible. That is, if notorque is being applied for a long duration, the wheels should not bespinning. Instant adaptation in step 84 includes taking the ratio of thedriven to non-driven wheels or driven wheels to reference vehicle at thetime the step is performed. This allows instant adaptation of the rate.In step 82, if the conditions are not met the dual rate adaptationillustrated in FIG. 2 is performed in step 86. After steps 84 and 86, anext loop is performed in step 88.

While particular embodiments of the invention have been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

1. A method of controlling a traction control system comprising:determining a wheel speed; determining when the wheel speed is above awheel speed threshold; determining when the wheel speed does not containa substantial high frequency content; determining a driven wheel speed;when the wheel speed is above the wheel speed threshold and the wheelspeed does not contain a substantial high frequency content, adaptingthe driven wheel speed to reference wheel speed ratio; and controllingthe traction control system in response to the driver wheel speed toreference wheel speed ratio.
 2. A method of controlling as recited inclaim 1, wherein adapting is performed when the traction control systemis not actuated.
 3. A method of controlling as recited in claim 1,wherein the driven wheel speed is average driven wheel speed.
 4. Amethod of controlling as recited in claim 1, wherein the driven toreference wheel speed ratio is the average driven wheel speed to theaverage undriven wheel speed ratio.
 5. A method of controlling asrecited in claim 1, wherein adapting is performed when the tractioncontrol system is not actuated, and a dual rate adaptation filter isapplied.
 6. A method of controlling as recited in claim 1, furthercomprising: determining when a driven wheel speed is larger than apre-determined function of reference wheel speed continuously duringtraction control activation for a pre-determined duration; and adaptingthe driven wheel speed to reference wheel speed ratio during theduration.
 7. A method of controlling a traction control systemcomprising: determining an instant driven wheel speed to reference wheelspeed ratio; determining a filtered wheel speed to reference wheel speedratio; comparing the instant driven wheel speed to reference wheel speedratio to the filtered driven wheel speed to reference wheel speed ratioto obtain a ratio difference; when the ratio difference is above athreshold, selecting the first filter constant, applying the firstconstant to an adaptation filter, resulting in a first filtered andadapted ratio and controlling the traction control system with theadapted ratio; when the difference is below the threshold, selecting thesecond filter constant, applying the selected constant to the adaptationfilter, resulting in a second filtered and adapted ratio and controllingthe traction control system with the adapted ratio.
 8. A method ofcontrolling as recited in claim 7 wherein the reference wheel speedcomprises an undriven wheel speed.
 9. A method of controlling as recitedin claim 7 wherein the reference wheel speed comprises a vehicle speed.10. A method of controlling as recited in claim 7 wherein the step ofselecting the first constant or selecting the second constant isperformed above a speed threshold.
 11. A method of controlling asrecited in claim 7 wherein the step of selecting the first constant orselecting the second constant is performed when the traction controlsystem is deactivated.
 12. A method of controlling as recited in claim 7wherein the step of selecting the first constant or selecting the secondconstant is performed when a driven wheel speed signal is above a noisethreshold.
 13. A method of controlling as recited in claim 7 wherein thedriven wheel speed signal is high pass filtered.
 14. A method ofcontrolling as recited in claim 7 wherein the step of selecting thefirst constant or selecting the second constant is performed when anon-driven wheel speed signal is above a noise threshold.
 15. A methodof controlling as recited in claim 7 wherein the non-driven wheel speedsignal is high pass filtered.
 16. A method of controlling as recited inclaim 1 further comprising determining a traction control duration;determining a current driven wheel speed; determining a currentreference speed; determining target wheel speed; when the tractioncontrol duration exceeds a first time limit and the driven wheel speedis greater than the target wheel speed continuously for a second timeperiod setting the adaptation ratio to the current driven wheel speeddivided by the current reference speed.
 17. A method of controlling asrecited in claim 12 wherein the current reference speed comprises anundriven wheel speed.
 18. A method of controlling as recited in claim 12wherein the current reference speed comprises a vehicle speed.
 19. Amethod of controlling a traction control system comprising: determininga reference speed; determining a driven wheel speed; determining a longterm filter value; determining a long term wheel speed ratio from thereference speed, the driven wheel speed and the long term filter value;determining a short term wheel speed ratio from the reference speed andthe driven wheel speed; operating the traction control system inresponse to the short term wheel speed ratio and the long term wheelspeed ratio.
 20. A method of controlling as recited in claim 19 whereinthe long term filter value comprises a long term average.
 21. A methodof controlling as recited in claim 19 wherein the traction controlsystem is an engine only traction control system.
 22. A method ofcontrolling as recited in claim 19 wherein the reference wheel speedcomprises an undriven wheel speed.
 23. A method of controlling asrecited in claim 19 wherein the reference wheel speed comprises avehicle speed.
 24. A method of controlling as recited in claim 19further comprising determining a traction control duration; determininga current driven wheel speed; determining a current reference speed;determining target wheel speed; when the traction control durationexceeds a first time limit and the driven wheel speed is greater thanthe target wheel speed continuously for a second time period setting theratio to the current driven wheel speed divided by the current referencespeed.